In optical data transmission, data may be transmitted by modulating the phase or the phase and the amplitude of an optical wavelength in dependence on the transmission data and in accordance with a constellation diagram of a respective modulation method. Each point of the constellation diagram represents a finite set of data bits that are to be transmitted. Depending on the set of data bits that are to be transmitted, the phase or the phase and the amplitude of the optical wavelength is changed such that the resulting signal corresponds to the respective point of the constellation diagram. Such a modulation method is a digital modulation method. An example for a phase modulation method is the phase-shift keying (PSK) method called quadrature phase-shift keying (QPSK), in which each point of the corresponding constellation diagram represents two bits. An example of a method for modulating the phase and the amplitude of the optical signal is the method called 16-Quadrature-Amplitude-Modulation (QAM), in which each point of the corresponding constellation diagram represents four bits.
The set of bits represented by a point of a constellation diagram is called a symbol. The rate, by which the phase of the wavelength is changed and by which therefore the symbol changes, is called the symbol rate.
In order to increase the data rate for transmitting data via such a modulation of a specific optical wavelength, a technique called Polarization Division Multiplexing (PDM) may be exploited:                a first optical signal of the specific wavelength and of a first polarization state is modulated in dependence on a first data stream,        a second optical signal of the same specific wavelength and of a second polarization state, which is orthogonal to the first polarization state, is modulated in dependence a second data stream, and        both optical signals are transmitted over a same optical fibre as a combined optical signal with a respective state of polarization that is the result of the first and the second polarization state.        
At a receiving side, the two optical signals generated by PDM may be obtained, by sampling the optical field resulting from the combined signal along two states of polarization that are orthogonal to each other.
For further increasing the data rate, the technique of PDM may be applied to optical signals of different optical wavelengths, and then these optical signals of different optical wavelengths are transmitted over a same optical fibre. This is called Wavelength Division Multiplexing (WDM).
When transmitted in an ideal optical fibre, the state of polarization of the combined optical signal generated by PDM would experience a rotation with respect to its value at the transmitter, such that the polarization state of the combined signal is rotated. At a receiving side, the two transmitted optical signals along with their respective states of polarizations forming the combined signal can be recovered after sampling the optical field of the received combined optical signal along two arbitrary orthogonal polarization states.
One effect occurring when transmitting an optical signal at a specific wavelength in a specific polarization state over a non-ideal fibre is the effect known as cross-polarization modulation. Cross-polarization modulation describes a generation of another optical signal at the same wavelength but with another polarization state that is orthogonal to the specific polarization state of the transmitted optical signal. Thus, due to cross-polarization modulation, the first optical signal may generate signal components of the polarization state of the second optical signal, which may lead to a degradation of the second optical signal, and vice versa.
The effect known as cross-phase modulation describes an impact of one optical wavelength onto the phase of another optical wavelength, when transmitted over a same optical fibre that is not ideal. Thus, when performing Wavelength Division Multiplexing (WDM) by transmitting multiple optical signals with respective wavelengths over a same non-ideal optical fibre, the different optical signals may degrade each others phases.
Cross-polarization modulation and cross-phase modulation are transmission distortions caused by nonlinearities of an optical fibre, such as for example the Kerr effect. The Kerr effect is arises from a change of the fibres refractive index, which in turn is caused by the signal power of the transmitted signal within the fibre. In the case of using the techniques of PDM, multiple signals are transmitted, wherein each signal power of a transmitted signal has an own random character. Therefore, the overall Kerr effect caused by the different signal powers also has a random character, and thus in turn the overall transmission distortions of cross-polarization and cross-phase modulation vary in time and have a random character. Furthermore, additional effects such as thermal stress or mechanical stress acting on the optical fibre may cause stress-induced birefringences of the optical fibre, which in turn further add to the overall random character of the transmission distortions.
When receiving a combined optical signal with a polarization state that is the result of two orthogonally polarized optical signals, it is a common procedure to generate two received time-discrete signals, by sampling the received combined optical signal in two polarization planes that are orthogonal to each other, wherein these polarization planes of sampling are not necessarily identical to the polarization states at which the two orthogonally polarized optical signals are received. The polarization planes of sampling may be rotated in relation to the polarization states of the two optical signals forming the combined optical signal. This rotation is compensated for, by filtering the two received time-discrete signals using a set of finite-impulse response (FIR) filters and thus generating two filtered time-discrete signals. The filter coefficients of the FIR filters are determined using a constant modulus algorithm (CMA). The two filtered time-discrete signals are then used for demodulating respective received time-discrete data signals from them.
The effects of cross-polarization and cross-phase modulation may lead to a degradation of the transmitted optical signals and thus to bit errors when demodulating data signals from the filtered time-discrete signals on the receiving side. Occurring bit errors might be compensated, by encoding data bits into a block of bits using a forward error correction (FEC) encoding algorithm on the transmitting side before modulating the optical signal, and then decoding on the receiving side the received block of bits according to the used FEC algorithm. A FEC algorithm is able to correct only a maximum number of bit errors per block.
It has been observed by the inventors, that when transmitting data using the above mentioned techniques in conjunction, the number of bit errors—or the bit error ratio (BER)—caused by cross-polarization and cross-phase modulation are not constant over time. A peak of bit errors—or a peak of the BER—is called a burst of bit errors. Such a burst in turn leads to FEC blocks, for which the number of correctable bits may be exceeded. Therefore, transmitted data bits of a data signal may remain uncorrected on the receiving side after FEC decoding.
It is therefore an aim of the invention to improve the known method of data transmission.
The document D1 discloses a system, in which polarization mode dispersion is introduced after previously modulating an optical signal using an optical sequence.
The document D2 discloses a system, in which in-phase and quadrature signal components of a single modulated optical signal with one single polarization state are delayed to each other, in order to achieve a decorrelation of these signal components for this single optical signal. The single optical signal with the single polarization state is then provided to a device that emulates polarization multiplexing.
This polarization multiplexing device takes the single optical signal with the single polarization state and creates two delayed versions of the single optical on two orthogonal polarizations planes, in order to emulate polarization multiplexing.
The document D3 discloses a system, in which dispersion compensation is carried out at a receiving device using a finite impulse response filter (FIR).